Radar time and distance which are defined as $\tau^\star = \frac{\tau^+ + \tau^-}{2}$
and $r^\star = \frac{c(\tau^+ - \tau^-)}{2}$, can be used in the context of general relativity to attribute time ...

Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ...

I recently read a metaphor for how higgs bosson gives mass to matter.
"The higgs boson creates a higs field which passes throug all matter in the universe and gives it mass like an objecting passing ...

Black hole theory involves space (or space-time), itself, being sucked into the black-hole, with the event horizon marking the point at which space/space-time is moving faster than the speed of light. ...

Usually relativistic time dilation effects require something traveling near the speed of light. But the high speed requirement can be replaced with a high distance requirement instead. Traveling away ...

The light cone of our galaxy is a lot different to the light-cone of a galaxy 5 bly away. Our Hubble volumes are much different. Everyone is born in/at a different space/time.
Can I see stars in my ...

Space is expanding and as we know space and time are intrinsically linked to be now known as spacetime.
What is happening to time during expansion? Is there more time, longer time or is the time part ...

I want to know if (hypothetically) a star appears out of nowhere at a certain distance (say 20 light seconds) away from me, how long will it take for me to get the feel of it's gravity? Will I know it ...

What is the need of 4th dimension in GR. Can we solve the problem without assuming 4th dimension. If we specify in three dimensional grid values of relative time in all the elements of grid, can we ...

we've been told that in General Relativity (GR), matter tells space how to curve and space tells matter how to move. But my question is, if 3 dimensional space was curved by matter then it should be ...

Every time I try to find the answer to this question I get redirected to different pages that ultimately do not end up answering my question. I have some understanding of Riemannian geometry but have ...

Let's suppose I have a spacetime manifold $M$. Let $p$ be a point on my manifold. Now I move from $p$ to some other point $p'$. Presumably I should have moved some "distance" right? How can I speak of ...

I do not know much about quantum physics. However, I do know it believes the world is discrete ( has quanta). This seems to contradicts the fact that we can create an object of length root 2 since you ...

There is a possibility for our universe to be the surface volume of a (higher-dimensional) hyperspace. So if this possibility is true,then is there also a possibility that the other galaxies we see ...

I'm a chemistry student interested in physics. Hope the question doesn't sound funny.
As opposed to Newton's gravity, which doesn't explain how gravity works, Einstein explained gravity as a result ...

The question I have is: Why is space (almost perfectly) flat in our neighbourhood? (I am disregarding the deviations due to the sun and the planets.)
Is it correct to say that space is (almost) flat ...

I've been wracking my brain trying to understand what "curved spacetime" really is, and I think replacing one dimension with the time dimension then drawing the world-lines through time was the "aha!" ...

I've read an analogy that finding iron-rich galaxies just 900 years after the Big Bang is like finding an old man in a crib in a nursery. We just recently found a supermassive black hole 12 billion ...

This came to me after reading that a black-hole that has the mass of the observable universe will also have an event horizon that covers the observable universe.
Since the definition of a black hole ...

How do we know the expansion of the universe is "of space" and not "in space" or "into space" or another less intuitive arrangement? For example, what implicit and explicit assumptions underlie the ...

Okay, so I am reading a book, "The Elegant Universe" by Brian Greene, which talks about motion and its effect on time.
Greene makes the point that time changes with motion by saying that if you have ...